Session Library differences between CI3 and CI4?

The concurrency part has to due with Ajax requests, you should always do a write close on the session
if your using Ajax requests.

Doesn't answer the question though.

Is the Session Handling Library in CI 4 brand new? Ie. Not the same as CI 3? If not, then perhaps the section for Concurrency in CI 4 Session doc should be rewritten so that it references the CI 3 rewrite instead of making it appear as if its a brand new library rewrite.